Recently, in order to keep up with the age of ubiquitous electronic devices, a technique that can realize high-density mounting (fine-circuit formation) at a low cost has been desired in the production of circuit wiring of electronic devices. A known example of such a technique is a method for forming conductive wiring, the method including printing a silver paste containing, as a component, silver particles having a nanometer size (hereinafter referred to as “nano-silver”) by a screen printing method to form a fine pattern, and then baking the silver paste at a low temperature of 150° C. or lower.
A printing method can provide inexpensive circuit wiring because of a reduction in the number of processes and the high throughput property thereof. In addition, if low-temperature baking at 150° C. or lower can be realized, it will be possible to use, as a substrate material that is an alternative to expensive polyimides which have been used to date, commodity plastics, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), which are inexpensive and have low heat resistance but are easily formed into a thin film, i.e., easily molded into a flexible film. Furthermore, the use of the silver paste containing nano-silver can realize highly fine patterning, and will contribute to the realization of high-density mounting. This is because an existing silver paste containing silver having a micrometer size may clog a screen mesh having a fine pitch necessary for highly fine printing, whereas the silver paste containing nano-silver does not have such a drawback.
Under these circumstances, various conductive pastes for printing have been proposed. Various conductive pastes for screen printing containing metal nanoparticles protected by a polymer compound containing a basic nitrogen atom, a deprotecting agent for the metal nanoparticles, and an organic solvent are known as conductive pastes for screen printing.
For example, Patent Literature 1 discloses a silver paste in which nano-silver having an average particle diameter of 0.1 μm or less is used as a silver component. However, it is necessary to bake this silver paste at 200° C. or higher in order to achieve a volume resistance of 10−5 Ωcm. Thus, it is difficult to print this silver paste on a plastic substrate having insufficient heat resistance.
Furthermore, Patent Literature 2 discloses a technique in which the baking temperature is decreased by irradiating of energy rays before a baking step. However, this technique has a problem in that a plastic substrate is degraded by the energy rays.
Thus, a conductive paste is not yet known with which circuit wiring having a lower volume resistance value can be formed on a plastic substrate having lower heat resistance and lower durability to energy rays, by low-temperature baking.